1. Field of the Invention
Though the present invention is an apparatus developed for an ink-jet recording head, it can be widely used as an apparatus for forming a conductive film of a small electric circuit or integrated circuit and moreover, performing fine printing in addition to the ink-jet recording head. The present invention relates to the improvement of the art disclosed in Japanese Patent Application Laid-open No. 57963/1997 (hereafter referred to as xe2x80x9colder applicationxe2x80x9d) previously applied by the present applicant.
2. Description of the Related Art
The present applicant disclosed a droplet ejection apparatus according to a new theory in the above older application. The droplet ejection apparatus comprises an main chamber having an inlet and an ejection aperture and pressurizing means for applying a pressure to the liquid introduced into the main chamber. The ejection aperture forms surface waves on the surface of the injection liquid contacting the air at the ejection aperture by the pressure and ejects droplets having a diameter smaller than that of the ejection aperture in accordance with the action of the surface waves. To form surface waves on the surface of the liquid at the ejection aperture, the sectional shape illustrated in FIG. 40 is disclosed. FIG. 40 is an illustration showing the structure of a ink droplet ejection apparatus. The droplet ejection apparatus is provided with an inlet 1, an ejection aperture 2, a vibration plate 3, a piezoelectric actuator 4, an main chamber 5, and an ink supply 6.
When providing a mechanical displacement for the vibration plate 3 driven by the piezoelectric actuator 4, the pressure of the ink stored in the main chamber 5 changes and surface waves are produced on the surface of the ink at the ejection aperture 2. The surface waves move from the circumference of the ejection aperture 2 to the central portion, interfere with each other at the central portion to increase their wave height, and resultingly droplets of the ink separate from the surface of the ink. The ink is fed to the injection chamber 5 from the ink supply 6 after passing through the inlet 1.
The above phenomenon is conceptually described below. When dropping a drop of water onto a stationary water surface, an annular surface wave expands centering about the drop-of-water fall point. A phenomenon just reverse to the above phenomenon occurs on the ink surface at the injection aperture 2 of the present invention. When producing surface waves bound for the center of the ejection aperture 2 from the circumference of the aperture 2, the waves concentrate on the center of the ejection aperture 2 and ink droplets separate from the ink surface.
FIG. 41 is a structural drawing for explaining the aperture portion of a printing apparatus provided with a plurality of ink droplet ejection apparatuses. As shown in FIG. 41, by arranging a plurality of ejection apertures 2 of droplet ejection apparatuses 141 to 14n and controlling the ink ejection of each ejection aperture 2, it is possible to print the paper passing through the front of the ejection aperture ? in the direction of the arrow. Thereby, it is possible to constitute the head of the printing apparatus.
An apparatus according to the new theory makes it possible to eject a droplet having a diameter smaller than that of an ejection aperture. Therefore, even if an ejection aperture having a large diameter is formed by roughly setting a machining accuracy, it is possible to perform high-resolution printing by ejecting small droplets. That is, it is possible to provide a high-resolution apparatus inexpensively and easily. Moreover, because it is possible to increase the diameter of an ejection aperture, clogging with ink does not easily occur, and an apparatus has a high adaptability to the surrounding environmental change. That is, available temperature range and humidity range are expanded. Moreover, there are superior features including the fact that requirements to the composition of a liquid are moderated and thereby, the liquid can be adapted to various types of inks.
The inventor of the present application et al. performed various tests on the droplet injection apparatus according to the new theory. Then, they confirmed through the tests that the droplet ejection apparatus according to the theory was considerably effective. As the standard of a practical printing apparatus, at least a resolution of approx. 300 dpi (dots per inch) or higher is required to print beautiful Japanese characters. In the case of the present invention, study has been progressed by aiming at the development of a practical printing apparatus having a resolution of 300 dpi or higher.
In this case, the most important problem to obtain a practical apparatus is to form surface waves on the surface of an ejection aperture instead of directly discharging an injection liquid from the ejection aperture. Moreover, another important problem is how to constantly stably form the surface waves under environmental conditions including practical temperature and humidity. To solve the problems, it is necessary to consider the following factors: (1) mechanical structures or shape of main chambers and aperture, (2) viscosity, surface tension, density, and other physical properties of liquid, and (3) art for controlling pressure to be applied to main chamber.
The first invention discloses a condition obtained as the result of performing many tests on the above Item (1) and apparatus structure according to the condition. It is an object of this invention to provide a compact, simple, and high-resolution droplet ejection apparatus. It is another object of this invention to provide a practical printing apparatus having a resolution of 300 dpi or higher. It is still another object of this invention to provide a droplet ejection apparatus which can be widely used as an apparatus for forming a conductive film of a small electric circuit or integrated circuit and moreover performing fine printing.
The second invention discloses a condition obtained as the result of performing many tests on the above Item (2) and an apparatus structure according to the condition.
It is an object of the second invention to provide an apparatus less influencing a liquid and capable of performing stable ejection even if the operating environmental temperatures of the apparatus are changed.
The third and the fourth inventions disclose a condition obtained as the result of performing many tests on the above Item (3) and an apparatus structure according to the condition.
The first invention is a droplet ejection apparatus comprising an chamber having an ejection aperture and pressuring means for applying a pressure to the liquid introduced into the chamber, in which the chamber is formed into a shape for forming surface waves on the surface of the liquid at the ejection aperture with the pressure and ejecting droplets having a diameter smaller than the diameter of the ejection aperture and whose sectional size vertical to the ejecting direction is decreased toward the ejection aperture, wherein the cross section of the chamber vertical to the injecting direction is circular or regular polygonal.
The first invention is characterized by forming the planar sectional shape of the chamber to be circular or regular polygonal. That is, for surface waves to be synthesized at the central potion of an ejection aperture, a circle or regular polygon is suitable for the shape of the ejection aperture. Because the ejection aperture is formed at an end of the wall surface of the chamber, it is proper to form the planar sectional shape of the chamber to be circular or regular polygonal.
It is preferable that the angle xcex8 formed between the wall surface and a plane vertical to the ejecting direction (see FIG. 1) is set to 65xc2x0 or less and the diameter D of the ejection aperture is set to a value 1.25 or more times larger than a desirable diameter of droplets to be injected from the ejection aperture. According to the results of tests, it is more preferable that the angle xcex8 is set to 60xc2x0 or less and 15xc2x0 or more.
The present inventor et al. could obtain the optimum values of the angle xcex8 and the diameter D to develop a practical apparatus for the droplet ejection apparatus of the older application. In general, the ink used for a droplet ejection apparatus has a viscosity of 1.5 to 5 cP in the case of a water-based ink, a viscosity of 8 to 15 cP in the case of an oil-based ink, and a viscosity of 8 to 15 cP in the case of a hot-melt ink. The surface tension ranges between 10 and 70 dyne/cm in any case. As the result of performing ejection experiments by using the above various types of inks, it is found that the angle xcex8 increases and it is impossible to form surface waves on the free surface of a liquid over 65xc2x0, that is, the liquid protrudes cylindrically. Moreover, it is found that, by decreasing the angle xcex8, it is possible to form surface waves at the ejection aperture. As the result of performing more minute examination, it is found that the phenomenon in which a liquid cylindrically protrudes hardly occurs by setting the angle xcex8 to a value smaller than 60xc2x00. That is, it is found that most of the pressure applied to the liquid is used to form surface waves and concentric preferable surface waves are efficiently formed by setting the angle xcex8 to a value smaller than 60xc2x0. The lower limit of the angle xcex8 is determined by the fact that strength or stiffness is decreased due to decrease of the wall thickness nearby an aperture in addition to the problems on the machining including the volume of an chamber and the relation with an adjacent ink supply. From the viewpoint of a practical structure, the lower limit is an angle xcex8 of approx. 15xc2x0.
Moreover, it is found that the diameter D requires a value 1.25 or more times larger than a desired diameter of a droplet to be ejected in order to eject droplets by making surface waves interfere each other. That is, if the diameter D is smaller than the above value, formed surface waves are held together due to the surface tension and thereby, it is impossible to form preferable surface waves. However, when the diameter D is larger than the above value, preferable surface waves are formed. Further, increasing the above value reduces the cost for machining an ejection aperture. However, by increasing the above value, it is necessary to consider that the distance from an adjacent ejection aperture is restricted, more amount of ink is evaporated, and formed surface waves are attenuated when they propagate on the surface. In the case of a practical structure, the upper limit of the above value is a value approx. three times larger than a desired maximum diameter of a droplet to be ejected.
If the wall surface of the chamber is displaced due to the applied pressure, formation of surface waves is rendered weak. That is, when decreasing the angle xcex8 of the wall surface in the chamber to less than 60xc2x0, most of the pressure applied to the liquid is used to form surface waves and surface waves are efficiently formed. However, because the wall thickness nearby the injection aperture decreases, the strength and stiffness of the wall surface are decreased. When the stiffness decreases, the vicinity of the edge of the ejection aperture is vertically displaced due to droplet ejection and a problem occurs that the surface-wave formation efficiency lowers or liquid injection becomes unstable.
Therefore, by forming the wall surface of the chamber like a knife edge, it is possible to increase the stiffness of the wall surface of the chamber. That is, it is possible to form the wall surface of the chamber so as to be flared forward the outside of the chamber from the middle though the wall surface slowly narrows toward the ejection aperture. In this case, the substantial ejection aperture is a portion where the diameter of the chamber is minimized.
Moreover, it is effective to set a reinforcing member for preventing the wall surface from being displaced due to the pressure applied to the injection chamber around the injection aperture. According to this structure, the wall surface of the injection chamber is not displaced due to the pressure and therefore, it is possible to effectively form surface waves. In this case, it is possible to form the aperture of the reinforcing member into any shape as long as the shape does not interrupt the formation of surface waves on the liquid level or the liquid ejection through the ejection aperture. The diameter of the aperture of the reinforcing member can be smaller than that of the ejection aperture as long as a droplet smaller than the diameter of the ejection aperture can effectively pass through the aperture of the member.
The second invention is a droplet ejection apparatus comprising an chamber having an injection aperture and pressuring means for applying a pressure to the liquid introduced into said chamber, in which the chamber is formed into a shape for forming surface waves on the surface of the liquid at the ejection aperture with the pressure and ejecting droplets having a diameter smaller than the diameter of the ejection aperture, and means for heating said liquid is included.
The second invention is characterized by including means for heating the injection liquid.
It is preferable that the heating means includes means for controlling the temperature of the liquid almost constantly. Moreover, it is preferable that the heating means is set so that the temperature of the liquid becomes higher than the practical maximum temperature of an apparatus.
It is possible to constitute a structure provided with an electric heater for heating the wall surface of the chamber. That is, it is also possible to constitute a structure in which the wall surface is made of a heat conducting member and a heater element contacting the heat conducting member is included or a structure in which the wall surface is constituted with an electric exothermic body.
Moreover, it is possible to constitute a structure in which an electric exothermic body is formed on the contact surface of the pressurizing means with the liquid.
Furthermore, it is possible to constitute a structure in which the heating means heats a head including a plurality of the chambers and a plurality of the pressurizing means.
To develop a practical apparatus for the droplet ejection apparatus of the older application, the present inventor et al. noticed that ejection characteristics were changed depending on the physical properties such as the surface tension and viscosity of a liquid. They confirmed that a liquid having lower viscosity and larger surface tension easily formed very small droplets and the speed of droplets to be ejected (hereafter referred to as droplet speed) could be increased. In general, a liquid changes in viscosity and surface tension depending on temperature. Therefore, it is found that droplets are stably ejected at desired droplet diameter and droplet speed by controlling the temperature of the liquid. Moreover, it is found that, even with changes in the environmental temperatures, stable droplet discharge can be maintained by controlling the temperature of the liquid.
The third invention is a droplet injection apparatus comprising an injection chamber having an ejection aperture and pressuring means for applying a pressure to the liquid introduced into the chamber, in which the chamber is formed into a shape for forming surface waves on the surface of the liquid at the ejection aperture with the pressure and droplets having a diameter smaller than the diameter of the ejection aperture, and a pulse to be applied to the pressurizing means is a single pulse having a pulse width xe2x80x9ctxe2x80x9d of 100 xcexcS or less.
The third invention is characterized by using a single pulse having a pulse width xe2x80x9ctxe2x80x9d of 100 xcexcS or less as the pulse to be applied to the pressurizing means when the diameter of the injection aperture is 1.25 or more times larger than a desired droplet diameter. According to the results of tests, it is preferable that the pulse width xe2x80x9ctxe2x80x9d is 50 xcexcS or less. The pulse width xe2x80x9ctxe2x80x9d can be set to various values. In this case, a pulse width is equivalent to the time until returning the liquid in the chamber after pressurizing it.
To develop a practical apparatus for the droplet ejection apparatus according the new theory disclosed in the older application, the present inventor et al. performed various tests on the value of the pulse width xe2x80x9ctxe2x80x9d to be applied to pressurizing means. That is, as described above, an ideal dot diameter on a printing medium at a desired resolution of 300 dpi requires approx. square root of 2 times valve larger than a dot pitch and this value corresponds to approx. 120 xcexcm. Moreover, it is possible to experientially recognize that the relation between dot diameter and droplet diameter on a chart depends on the characteristic of a printing medium or the speed of an ejected droplet. Moreover, it is experimentally found that the speed of an ejected droplet cannot greatly be changed in accordance with the composition of a liquid as long as following the new theory. That is, it is found that, by forming surface waves so as to separate droplets from the liquid surface, the speed of the ejected droplets becomes a nearly constant value (e.g. approx. 3 to 10 m/S when heating a typing ink used for experiments to a temperature approx. 30xc2x0 C. higher than room temperature) and the practical speed of the ejected droplets becomes approx. 4 m/S even if changing the energy to be applied to pressurizing means or using an ejection aperture having a different diameter. To form a printing dot having a diameter of 120 xcexcm on coated paper under the above condition, it is necessary to eject a droplet having a diameter of approx. 60 to 70 xcexcm. It was observed how droplet diameters changed by changing the pulse width xe2x80x9ctxe2x80x9d to be applied to pressurizing means in order to discharge an ink droplet having a diameter of approx. 60 to 70 xcexcm. As a result, it is found that an almost desired droplet diameter is obtained by setting the pulse width xe2x80x9ctxe2x80x9d to 100 xcexcS or less and moreover, it is found that it is more preferable to set the pulse width xe2x80x9ctxe2x80x9d to 50 xcexcS or less.
It is possible to set the pulse width xe2x80x9ctxe2x80x9d to various values. Thereby, it is possible to correspond to the temperature of ink and moreover, the change of environmental conditions and change practical droplet diameters by changing the pulse width xe2x80x9ctxe2x80x9d.
The fourth invention is a droplet ejection apparatus comprising an chamber having an ejection aperture and pressuring means for applying a pressure to the liquid introduced into the chamber, in which the chamber is formed into a shape for forming surface waves on the surface of the injection liquid at the injection aperture with the pressure and ejecting droplets having a diameter smaller than the diameter of the ejection aperture, the pressurizing means is provided with an electric-signal generation circuit and an piezoelectric actuator which is driven by an output of the electric-signal generation circuit and whose mechanical-displacement output is applied to the liquid in the chamber, and a filter circuit for selectively passing a frequency component suited to form the surface waves is connected to a circuit between the output of the electric-signal generation circuit and the piezoelectric actuator.
The fourth invention is characterized in that the pressurizing means is provided with an electric-signal generation circuit and a piezoelectric actuator which is driven by the output of the electric-signal generation circuit and whose mechanical displacement output is applied to the liquid in the chamber and a filter circuit for selectively passing frequency components suited to form the . surface waves is set in the circuit between the output of the electric-signal generation circuit and the piezoelectric actuator. It is preferable that the frequency components are sine-wave pulses.
In this case, a sine-wave pulse is defined as a pulse waveform having a very narrow frequency distribution included in a pulse signal.
It is preferable that the electric-signal generation circuit is a pulse generation circuit for generating a triangular pulse, rectangular pulse, or trapezoidal pulse and the filter circuit uses a low-pass filter. The low-pass filter can be realized by, for example, a CR filter.
To develop a practical apparatus for the droplet injection apparatus of the older application, the present inventor et al. noticed that a sine-wave pulse was most suitable for a waveform for pressurizing the ink in an injection chamber. The above fact was obtained by experimentally confirming that it was possible to make surface waves with arranged phases interfere each other at the center and perform the most stable discharge on droplet diameter and droplet-speed because the frequency component of a sine-wave pulse was single.
Therefore, they attempted to directly generate a sine-wave pulse by an electric-signal generation circuit. However, it is found that a synthesizer circuit or the like is necessary to generate a single sine-wave pulse and the cost increased. Therefore, it is found that a waveform close to a sine-wave pulse which is a basic wave can be obtained by generating a proper triangular, rectangular, or trapezoidal pulse by the electric-signal generation circuit and passing the pulse through a filter circuit comprising a low-pass filter instead of directly generating the sine-wave pulse. Thereby, it becomes possible to constitute the practical apparatus with simple and inexpensive circuits compared to the case of directly generating a sine-wave pulse by the electric-signal generation circuit.